The present invention relates to centrifugal pumps in general, and more particularly to improvements in centrifugal pumps of the type wherein the casing embodies a shell having a cylindrical or substantially cylindrical periphery, especially a shell which is a forging.
It is already known to employ casings which embody cylindrical shells in high pressure centrifugal pumps. As a rule, a centrifugal pump will be classified as a high pressure pump if its nominal total head is between 200 and 1200 meters. Such pumps can be used with advantage in pressure boosting as well as in many other plants. For example, high pressure centrifugal pumps find frequent use in power plants as well as (and especially) in many types of nuclear reactor plants. In such plants, the safety regulations are extremely stringent so that each and every component of the pump must be tested with a high degree of accuracy and must meet high standards prior to acceptance by the persons in charge. In many instances, testing involves resort to sources of ultrasonic radiation and/or penetrative radiation (e.g., X-ray equipment). The examination embraces all or nearly all components of a high pressure centrifugal pump, especially welded seams and those parts (such as the piping and casing) which must stand pronounced pressures for extended periods of time.
Presently known centrifugal pumps wherein the casings or housings embody cylindrical or substantially cylindrical shells surrounding one or more stages and having inlet and outlet means for conveyed fluid media exhibit a number of serious drawbacks. If the shell is to be welded to a tubular element (e.g., a pipe which is to constitute the inlet or the outlet) in a simple and convenient way (such as by resorting to a ring-shaped welded seam which connects a flat end face of the pipe to a similarly flat surface of the shell), the latter must be subjected to a number of complex machining operations in order to provide its periphery with one or more outwardly extending tubular nipples having flat end faces for abutment against the end faces of pipes prior to start of the welding operation. The cost of forged nipples is often prohibitive so that such technique failed to meet widespread acceptance in the industry.
Another serious drawback of many presently known centrifugal pump casings with cylindrical shells is that, if the aforediscussed nipples are to be omitted, the making of a welded seam between the end face of a tube and the cylindrical periphery of the shell involves extensive manual work and the quality of the resulting welded seams (which resemble distorted ellipses) is inferior (or normally inferior) to that of machine-made ring-shaped welded seams between two flat or nearly flat surfaces. Moreover, the testing of welded seams which are not flat rings is difficult, unreliable and time-consuming.
A further drawback of conventional centrifugal pumps having casings which embody cylindrical shells constituting one-piece forgings is that such forgings cannot be readily attached to conventional foundations or supports except by resorting to additional welded seams (each of which necessitates complex examination prior to acceptance of the pump) and/or to bolts, screws or analogous threaded fasteners which do not permit for any movement of the foundation relative to the shell and/or vice versa. Such drawbacks arise regardless of the nature of the foundation, i.e., regardless of whether the shell is secured to two or more arms of a foundation or is welded to brackets which are welded to the base plate of the foundation.